This invention relates to an access method for bridge transfer in data transmission.
A conventional data transmission system using wireless communication technology has been proposed in IEEE 802.11. This document was written by Wim Diepstraten, Phil Belanger and Greg Ennis, and titled "Distributed Foundation Wireless Medium Access Control", IEEE document, DOC:IEEE P802 11-93/190 (referred to as Reference 1).
The above data transmission system executes the procedure to cause an addressed receiving station to always return a response to the transmitting station (except broadcast communications).
This system defines two types of network, Ad-Hoc Network and Infrastructure Network. The Infrastructure Network is a system provided with Access Points connected to wired networks. In this system, if the transmitting station cannot transmit signals directly to the destination station, it transmits the signal addressed to the destination station to the Access Point, from where the signal is transferred to the destination station.
FIG. 14 shows detailed operation of the access method described in Reference 1.
It is provided with a procedure to exchange control information 81, 82, and 84 when transmitting data 83.
The control information 81 is used for identifying the channel connectability to the destination terminal. The control information 82 is used for responding to the identification made by the control information 81. The control information 84 is used for acknowledging the data reception. The control information 81 and 82 contain information as to how long each terminal uses the channel afterward.
Each time interval 816, 812, and 813 has the shortest time interval used as a gap between exchange procedures.
Data 823 is subjected to the succeeding procedure started by another terminal after confirming termination of the transmission procedure.
The control information 86 is used by a relay station for the entire controlling. The preceding time interval 815 is longer than the time intervals 816, 812, and 813.
At a starting point of the exchange procedure, each time interval 811 and 814 spent for confirming connectability (channel occupation status) on the channel for transmission of control information 81 and data 823 is longer than the time interval 815. Each time interval 811 and 814 at the respective terminals is set to have a random time interval which is longer than at least the time interval 815. Upon receiving the control information 81 and 82, other terminals stop transmitting during the period for using the channel contained therein.
Each field 821 and 822 in this figure shows the occupation time interval contained in the control information 81 and 82, respectively.
Reference 1 describes the following program showing detailed operations.
In this program, the term RTS corresponds to the control information 81, CTS to the control information 82, data for a unicast to the data 83, Ack to the control information 84, DIFS to the time intervals 811 and 814, SIFS to time intervals 816, 812, and 813, and NAV to fields 821 and 822, respectively.
Each RTS, CTS, and Ack is required to have CRC added thereto. Backoff functions in suspending the process to move to succeeding operation for the period equivalent to the value derived from random number independently owned by each terminal.
The timer T1 in the following program indicates the time interval from transmitting the control information 81 to starting transmission of the data 83. The timer T3 indicates the time interval from transmitting the data 83 to terminating the transmission of the control information 84.
Algorithm for transmitting station:
When transmitting a unicast MPDU using RTS/CTS option:
If medium free (No NAV and no CS) longer than DIFS, then transmit RTS.
Else defer until DIFS gap is detected and go into backoff.
If CTS is received within T1 after RTS, then transmit the DATA after SIFS.
Else go into Retransmit.sub.-- Backoff.
If Ack not received within T3, then go into Retransmit.sub.-- Backoff.
When transmitting a unicast MPDU not using the RTS/CTS option:
If medium free (No NAV and no CS) longer than DIFS, then transmit DATA.
Else defer until DIFS gap is detected, and go into backoff.
If Ack not received within T3 then go into Retransmit.sub.-- Backoff.
When transmitting a Broadcast/Multicast MPDU:
If medium free (No NAV and no CS) longer than DIFS, then transmit DATA.
Else defer until DIFS gap is detected, and go into backoff.
Algorithm for receiving station:
If RTS frame is detected but station is not the destination, Then:
Update the NAV with the Duration information and start a T1 timer.
Else
Return a CTS frame when medium free (No NAV and no CS) after SIFS.
If T1 timer expires, and CS is not active at that time, then clear the NAV.
If station is the destination of a unicast DATA frame, Then;
Transmit Ack after SIFS when CRC was correct.
In the conventional system of the aforementioned Reference 1, it is necessary to previously identify whether or not signals can be directly transmitted so as to determine the need to transfer the signal to the Access Point before transmitting signals to the destination station.
However channel of the wireless communication has instable nature, and each condition of the individual terminals is likely to change, such as being mobile, depending on the occasional environmental change. The aforementioned problems will make it difficult to update the information at every occasion.